This invention relates generally to a method and apparatus for providing distributed ignition of a combustion engine and, more particularly, to a method and apparatus for controlling the timing and amount of a pilot fuel injected into a combustion engine for distributed ignition.
Low cetane, i.e., high octane, fuels, such as natural gas, have several advantages over other hydrocarbon fuels that are combusted in internal combustion engines. For example, natural gas is less expensive relative to other hydrocarbon fuels. Moreover, natural gas bums cleaner during operation of the internal combustion engine relative to other hydrocarbon fuels. By burning cleaner, a reduced amount of combustion byproducts such as carbon monoxide, oxides of nitrogen, and hydrocarbons are released into the environment during engine operation. In addition, because lubricants of the internal combustion engine become contaminated with combustion byproducts over time, the production of a reduced amount of combustion byproducts results in less contamination, thereby increasing the useful life of the lubricants.
One type of internal combustion engine is a diesel engine. Diesel engines combust fuel by compressing a mixture of air and fuel to a point where the fuel is ignited by heat which results from such compression. When natural gas is used as a fuel in a diesel engine, the natural gas does not readily ignite as it is compressed. In order to overcome this problem, an ignition source is provided to ignite the natural gas. The ignition source may be provided by a spark plug similar to those used in spark ignition engines. However, in certain types of diesel engines, e.g., dual fuel engines, the ignition source is provided by injecting a small amount of pilot fuel, such as diesel fuel, into a mixture of air and natural gas (or other gaseous fuel). As the mixture of air, natural gas and pilot fuel is compressed, the pilot fuel ignites, which in turn provides a diesel type ignition of the natural gas.
A disadvantage associated with using pilot fuel as an ignition source is the resulting generation of an increased amount of oxides of nitrogen (NOx). In particular, the ratio of air to the combination of natural gas and pilot fuel in the combustion chamber varies with the proximity to the injected streams of pilot fuel. Rich mixtures are created near the location of injection of pilot fuel, while lean mixtures are created further away from the location of the injection. Combustion of the rich mixtures tend to produce more NOx than does the combustion of the lean mixtures.
One way to reduce the amount of NOx produced during the combustion process is to create a lean homogeneous mixture of air, natural gas and pilot fuel throughout the combustion chamber prior to ignition of the pilot fuel. Because the homogeneous mixture is lean throughout the entire combustion chamber, only lean mixtures are combusted. Combustion of only lean mixtures produces a lesser quantity of NOx than does combustion of a combination of rich mixtures and lean mixtures.
In commonly-owned U.S. Pat. No. 6,095,102, Willi et al. (Willi) discloses a method for injecting a quantity of pilot fuel into a combustion chamber having a supply of gas/air mixture. The pilot fuel is injected during the compression stroke in the range from about 21 degrees to 28 degrees before top dead center (BTDC) and is used to provide distributed ignition of the gas/air mixture. Willi discloses that injection of the pilot fuel in advance of what has been typically done in the industry, e.g., from 5 to 20 degrees BTDC, provides for a homogeneous mixture of the pilot fuel with the main portion of the gas and air. Furthermore, Willi discloses that the exact desired timing of the injection is determined by sensing the amount of NOx in the exhaust stream during each subsequent exhaust stroke and varying the timing until an optimal level of NOx is attained.
It has been found that, since Willi""s initial disclosed method, variations in engines and engine operating conditions result in situations in which the optimal desired timing of the pilot fuel injection resides outside of the 21 to 28 degree BTDC range during the compression stroke. Furthermore, sensing the level of NOx and responsively varying the pilot injection timing does not always yield the best results. For example, optimal results may be achieved by varying the timing of the pilot fuel injection as well as the amount of pilot fuel injected. This can only be accomplished by determining parameters other than merely sensing NOx, and responsively controlling both the timing and the amount of the pilot fuel injection.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention a method for injecting pilot fuel in a combustion engine is disclosed. The method includes the steps of determining a load of the engine, determining a desired injection timing of the pilot fuel and a desired quantity of pilot fuel to be injected as a function of a desired homogeneous distribution of the pilot fuel based on the engine load, and adjusting the injection timing and quantity of the pilot fuel to the desired values.
In another aspect of the present invention a method for providing distributed ignition of a combustion engine is disclosed. The method includes the steps of introducing a quantity of fuel/air mixture into a combustion chamber of the engine, determining an operating load of the engine, determining a desired injection timing of a pilot fuel and a desired quantity of the pilot fuel to be injected as a function of a desired homogeneous distribution of the pilot fuel with the fuel/air mixture based on the engine load, and injecting the pilot fuel at the desired time.
In yet another aspect of the present invention an apparatus for providing distributed ignition of a combustion engine is disclosed. The apparatus includes a cylinder assembly which includes (1) an engine block having a piston cylinder defined therein, (2) an engine head secured to the engine block, and (3) a piston which translates within the piston cylinder, wherein the engine block, the engine head, and the piston cooperate to define a combustion chamber. The apparatus further includes an intake port positioned in fluid communication with the combustion chamber during intake of a primary fuel and air mixture, a fuel injector positioned in the engine head and operable to inject pilot fuel into the combustion chamber during a compression stroke of the engine, an engine load determining device, and a controller which receives information from the engine load determining device and responsively determines a desired injection timing of the pilot fuel and a desired quantity of pilot fuel to be injected based on a desired homogeneous distribution of the pilot fuel.